1. Field of the Invention
The present invention relates to a toroidal continuously variable transmission, and more specifically, to a toroidal continuously variable transmission designed for use in vehicles such as automobiles.
2. Description of the Related Art
As a toroidal continuously variable transmission 20, the one shown in FIG. 1 has been known.
Inside a housing (not shown), an input disk 1 and an output disk 2 are coaxially arranged so as to face each other. An input shaft 3 extends through the axis of the toroidal transmission including the input and output disks 1 and 2. A loading cam 4 is disposed at one end of the input shaft 3. The loading cam 4 is configured to transmit the driving force (torque) of the input shaft 3 to the input disk 1 through a cam roller 5.
The input and output disks 1 and 2 have a substantially similar shape and are arranged symmetric. The surfaces of these disks facing each other are toroidal surfaces jointly forming a substantial semicircle sectional view taken axially. A pair of power roller bearings 6 and 7 for transmitting power are arranged in toroidal cavities defined by the toroidal surfaces of the input and output disks 1 and 2, in such a way that the bearings 6 and 7 be in contact with the input and output disks 1 and 2.
The power roller bearing 6 includes a power roller 6a which rolls on the toroidal surfaces of the input and output disks 1 and 2 (and which corresponds to an inner ring of the power roller bearing 6), an outer ring 6b and a plurality of rolling members (steel balls) 6c. The other power roller bearing 7 is composed of a power roller 7a which rolls on the toroidal surfaces of the input and output disks 1 and 2 (and which corresponds to an inner ring of the power roller bearing 7), an outer ring 7b and a plurality of rolling members (steel balls) 7c. 
In other words, the power roller 6a also functions as the inner ring, which is a structural component of the power roller bearing 6, and the power roller 7a also functions as the inner ring, which is a structural component of the power roller bearing 7. In this structure, the power roller 6a is rotatably coupled to a trunnion 10 by means of a pivot 8, the outer ring 6b and the rolling members 6c, and at the same time obliquely supported around a pivot axis O, which is the center of the toroidal surfaces of the input and output disks 1 and 2.
Likewise, the power roller 7a is rotatably coupled to a trunnion 11 by means of a pivot 9, the outer ring 7b and rolling members (steel balls) 7c, and at the same time obliquely supported by a pivot axis O, which is the center of the toroidal surfaces of the input and output disks 1 and 2. A lubricant having a large viscosity or friction resistance is supplied onto the contact surfaces of the input disk, output disk 1, 2 and the power rollers 6a, 7a. A driving force is exerted on the input disk 1 and transmitted to the output disk 2 through a lubricant film and the power rollers 6a and 7a. 
The input and output disks 1 and 2 are independent of the input shaft 3 with a needle 12 interposed between them (that is, the input disk 1 and output disk 2 are not directly affected by the movement of the rotating shaft 3). The output shaft 14, which is arranged in parallel to the input shaft 3 and rotatably supported by a housing (not shown) via angular members 13, is connected to the output disk 2.
In the toroidal continuously variable transmission 20, the driving force of the input shaft 3 is transmitted to the loading cam 4. When the driving force is transmitted and rotates the loading cam 4, the rotation force is transmitted to the input disk 1 through the cam roller 5, thereby rotating the input disk 1. The power generated by the rotation of the input disk 1 is transmitted to the output disk 2 by way of the power roller 6a and the power roller 7a. As a result, the output disk 2 rotates together with the output shaft 14.
At the time of transmission, the trunnions 10 and 11 are moved a little toward the pivot axes O. When the trunnions 10 and 11 are moved toward the axes, the intersection between the axis of rotation of the power rollers 6a and 7a and the axis of the input and output disks 1 and 2 is shifted slightly from the original position. As a result, the circumferential velocity of the rotation of each of the power rollers 6a and 7a and that of the rotation of the input disk 1 become off balance, and the component force of the torque of the input disk 1 makes the power rollers 6a and 7a to rotate obliquely around the pivot axes O. As a result, the power rollers 6a and 7a obliquely moves on the curved surfaces of the input and output disks 1 and 2.
Since the power rollers 6a and 7a are obliquely rotated on the curved surfaces of the input and output disks 1 and 2, a velocity ratio changes, with the result that deceleration and acceleration are performed. An example of a conventional toroidal continuously variable transmission having such a structure is shown in Jpn. UM. Appln. Publication No. 2-49411.
As the input disk, output disk and power roller bearing, those formed of AISI52100 (JIS SUJ2 corresponding to high carbon chromium steel) are conventionally known as described in xe2x80x9cNASA Technical note NASA ATN D-8362xe2x80x9d.
The present inventors have filed an invention (Jpn. Pat. Appln. KOKAI Publication No. 7-71555) in which the input disk, output disk and the power roller bearing are subjected to surface treatment such as carburization or carbonitriding processing to improve a resistant life. Alternatively, the surface treatment is limited to carbonitriding in Jpn. Pat. Appln. KOKAI Publication No. 9-79336, in which the nitrization amount and residual austenite are defined.
In the toroidal continuously variable transmission, the input disk, output disk, power roller, and loading cam repeatedly receive an extremely large bending stress and shearing stress as compared to machine parts such as a gear, which generally receives stress repeatedly. For this reason, it has been proposed that these disks, power roller, and cam disk should be formed of a material having a high fatigue strength and treated with heat to improve the fatigue strength.
For example, in Jpn. Pat. Appln. KOKAI Publication No. 7-71555, the input disk, output disk, power roller and power roller are formed so as to have a hardened layer whose effective depth is 2.0 mm or more and 4.0 mm or less. In Jpn. Pat. Appln. KOKAI Publication No. 7-286649, shot peening is performed after the heat treatment. In Jpn. Pat. Appln. KOKAI Publication No. 11-141638, assuming that dynamic maximum shearing stress is generated at a position Z0, the hardness of the hardened layer at a position satisfying 3.0 Z0 to 5.0Z0 is set at 650 or more.
However, recently, it has been demanded that the toroidal continuously variable transmission should be further miniaturized and the output of transmission torque be further increased. Therefore, mechanical parts are required to have higher fatigue strength than conventional ones.
The present invention has been attained to overcome the aforementioned problems. The present invention is to provide a toroidal continuously variable transmission having a long life and capable of suppressing the peeling-off of the traction surface of the toroidal surface formed of an input disk and an output disk and of the traction surface of a power roller and simultaneously improving the lives against fatigue cracking of the input disk, output disk, and power roller by constructing the toroidal continuously variable transmission such that at least one of the input disk, output disk, and the inner ring of the power roller bearing is formed of an alloy steel and subjected to either carburizing polishing and polishing or carbonitriding processing and polishing, and at least one of the input disk, the output disk, the inner ring of the power roller bearing has a hardened layer of Hv 653 having a depth 2.0 mm or more and 4.0 mm or less from the surface.
To attain the aforementioned object, the toroidal continuously variable transmission comprising an input disk attached to an input shaft, an output disk attached to an output shaft, and a power roller bearing including an inner ring, an outer ring, and a plurality of rolling elements, the inner ring engaging with the input and output disks to transmit power from the input shaft to the output shaft,
characterized in that at least one of the input disk, output disk, and an inner ring of a power roller bearing is formed of an alloy steel and subjected to either carburizing processing and grinding or carbonitriding processing and grinding and at least one of the input disk, output disk and inner ring of the power roller bearing has an effective hardened layer of Hv 653 having a depth of 2.00 mm or more and 4.00 mm or less from the surface.
A toroidal continuously variable transmission according to claim 2, characterized in that a grain size in the surface is 8 or more and a grain size in the core is 4 or more.
A toroidal continuously variable transmission according to claim 3, characterized in that the alloy steel contains 0.010 to 0.050% of Al and 0.005 to 0.030% of N.
A toroidal continuously variable transmission according to claim 4, characterized in that the alloy steel contains 0.015 to 0.035% of Al and 0.005 to 0.020% of N.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.